build: fix leaderboard script checking out wrong repo on forks

.github/workflows/leaderboard-submission.yml

@@ -27,6 +27,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           path: _pr_submission
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
           ref: ${{ github.head_ref }}
 
       # copy submission files over to main repo